The present invention relates to a vehicular power steering system, and more specifically to power steering systems equipped with means for controlling steering effort.
In general, resistance to steering decreases as the speed of a vehicle increases. On the other hand, the lateral acceleration of a vehicle during a turn increases as the vehicle speed increases. At higher speeds, therefore, a power steering system tends to make the steering too light and unstable. Therefore, power steering systems are often equipped with steering effort control means which provides the full power assistance for parking but reduces the degree of assistance at higher speeds by reducing the supply of hydraulic fluid to the power cylinder as the vehicle speed increases. In this case, however, there is a possibility that, when the steering wheel is turned at a high angular velocity, the fluid supply becomes short of the quantity required by an increase of the power chamber volume due to movement of the power piston of the power cylinder. Insufficient fluid supply makes the power cylinder inoperative temporarily, so that the steering becomes abruptly heavy. This condition is very dangerous. Furthermore, in conventional steering effort control systems, the lateral acceleration of a vehicle is not taken into account properly. When the driver begins to turn the steering wheel, he is not provided with a feel in the form of steering effort change that the lateral acceleration is going to increase, so that an unskilled driver tends to make an error in vehicle control by turning the steering wheel excessively.
It is an object of the present invention to provide a vehicular power steering system having a steering effort control system which is capable of controlling properly the degree of power assistance in accordance with the speed of the vehicle, the angular velocity of the steering wheel, and the lateral acceleration of the vehicle.